The present invention pertains to radars, and especially to the protection of radars against rain clutter, i.e. against parasitic echos from a simple reflection such as reflections on a plane or spherical surface.
In the prior art, this parasitic clutter is reduced by using radars in which the transmission and reception are done in circular polarization, with the same polarization direction used for transmission and reception. As compared with a radar working in rectilinear polarization, an attenuation of 15 to 20 dB is provided for these parasitic echos This attenuation is not always sufficient for it is indispensable to achieve greater rejection of echos of the rain clutter type. This is especially true for airborne radars which guide aircraft at minimum altitudes.
It is also known that with radar transmission in right-hand (or left-hand) circular polarization and with the same polarization at reception according to a first measurement, a useful echo from several reflections (ground or aircraft echo) is slightly attenuated and a parasitic echo (rain clutter) is attenuated by 15 to 20 dB by the polarizer. By contrast if, in a second measurement, all things being equal, the polarization at reception is reversed, the useful echo is slightly attenuated by about one decibel as compared with the first measurement, and the parasitic echo no longer undergoes the attenuation of 15 to 20 decibels It was on this basis that the European patent application of German origin No. 0 162 258 distinguishes rain clutter and therefore eliminates them by means of a transmitter/receiver aerial in right-hand circular polarization, for example, and by means of two ancillary receiving aerials, one in right hand circular polarization and the other in left-hand circular polarization. A comparison of the levels received makes it possible to distinguish between rain clutter This is an expensive device because it calls for two ancillary aerials. At best, the cost of this device can be reduced by using only one ancillary aerial.